Fire fighting vehicles with long extension ladders are equipped with outriggers on each side which are extended outwardly to provide stability when the ladders are raised, extended and rotated and the center of gravity of the ladder and the weight carried by the ladder is outward of the side of the fire fighting vehicle. With the outriggers fully deployed, the raised, extended ladder can be rotated to a desired position by an operator in the carriage at the top of the ladder and/or an operator at the base of the ladder. The fire fighting vehicle is designed so that the ladder can be safely operated within the design range and load limitations with the outriggers fully deployed.
Typically, the controls for rotating the ladder have an interlock which prevents rotating the ladder unless the outriggers have been fully deployed but which can be bypassed when the available space does not permit full deployment of the outriggers and the situation requires rotating the ladder. The operator simply actuates a manual over-ride switch which permits manual control of rotating the ladder to the desired position even with the outriggers not fully extended. Such arrangements place complete judgment and control of the rotation of the ladder with the operator which can lead to serious accidents. For example, if the ladder is rotated to a side where the outrigger is not fully extended, shifting the center of gravity can cause the fire fighting vehicle to capsize. The same result can occur if the rotating ladder approaches a side where the outrigger is not fully deployed at a high rate of rotational speed and is suddenly stopped.
To overcome this shortcoming of manual control of rotation of the ladder, it has been suggested to incorporate a control feature by which the operator is assisted during the manual control of the ladder. By sensing the rotation of the ladder during manual control operation with, for example, proximity switches and target plates, the direction of rotation and the approach to various critical points can be sensed, so that the ladder will be rotated only in the direction of an extended outrigger and the rate of rotational movement of the ladder can be reduced to prevent overshooting the critical point. In effect, manual control of the rotational movement of the ladder is over-ridden.
Proximity switches and target plates have been unsatisfactory in that the water being applied to fight a fire and debris affect the condition and operation of such devices. During cold weather when the water turns to ice, proximity switches and target plates can lock. In addition, proximity switches and target plates are difficult to install and calibrate.